Developmentally regulated transcription factors drive developmental gene programs that result in embryo formation and the birth, proliferation, growth, migration, and differentiation of the cells that eventually make up the different tissues of the body. This involves the expression and repression of many genes including those whose protein products act as regulators of this process as signal molecules. When the signal proteins are secreted, they may act both as paracrine signals between different cells, including on stem cells, and as autocrine signals on the same cells that produce the signal molecule. When the protein is not secreted, but rather inserted into the cell membrane, it may contribute to cell-cell interactions.
In the case of the developing nervous system, multiple secreted and non-secreted signal molecules expressed at different times and in different spatial locations are involved in: (i) determining the induction of the neural plate; (ii) regionalization of the neural tube along dorsoventral and anteroposterior axes; (iii) generation of neurons and glia from multipotent precursors (neuronal determination); (iv) determination of survival or apoptotic cell death; (v) migration of neurons; (vi) differentiation and regional patterning of neurons; (vii) neurite outgrowth and axon guidance; (viii) formation of specific synaptic connections between neurons, and (ix) determining neuronal-glial interactions.
Some of these signal molecules may be re-expressed in the adult after injury, or the failure of such re-expression may relate to the failure of mature neurons to survive, grow, or regenerate after injury. Some of the signal molecules may act in pathological situations to either promote or suppress abnormal growth or function. These signal molecules, acting on specific transmembrane receptors, may serve as neuronal determinants, survival factors, growth factors, guidance cues, or differentiation factors, and many may have potential therapeutic roles as biological agents beyond their specific involvement in development. Such factors can have biological activity both in vivo and for maintaining cultured cells in vitro, or for converting pluripotent stem cells into specific neuronal or non-neuronal subtypes. Similarly, mimicking the action of these signal molecules by activating their membrane bound receptors or the intracellular signal transduction pathways coupled to their receptors, may also have therapeutic potential.